The following description of the background of the invention is provided to aid in understanding the invention, but is not admitted to describe or constitute prior art to the invention.
Cellular signal transduction is a fundamental mechanism whereby external stimuli that regulate diverse cellular processes are relayed to the interior of cells. One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of tyrosine residues on proteins. The phosphorylation state of a protein is modified through the reciprocal actions of tyrosine kinases (TKs) and tyrosine phosphatases (TPs).
Receptor tyrosine kinases (RTKs) belong to a family of transmembrane proteins and have been implicated in cellular signaling pathways. The predominant biological activity of some RTKs is the stimulation of cell growth and proliferation, while other RTKs are involved in arresting growth and promoting differentiation. In some instances, a single tyrosine kinase can inhibit or stimulate, cell proliferation depending on the cellular environment in which it is expressed. Schlessinger and Ullrich, Neuron, 9(3):383-391, 1992. The platelet derived growth factor receptor (PDGF-R) and the role of its ligand (i.e., PDGF) in cancer are described in International Patent Application WO 95/19169, published Jul. 20, 1995, incorporated herein by reference in its entirety including any drawings.
RTKs are composed of at least three domains: an extracellular ligand binding domain, a transmembrane domain and a cytoplasmic catalytic domain that can phosphorylate tyrosine residues. Yarden and Ullrich, Ann. Rev. Biochem. 57:443-478, 1988. Ligand binding to membrane-bound receptors induces the formation of receptor dimmers and allosteric changes that activate the intracellular kinase domains and result in the self-phosphorylation (autophosphorylation and/or transphosphorylation) of the receptor on tyrosine residues. Their intrinsic tyrosine kinase is activated upon ligand binding, thereby initiating a complex signal transduction pathway that begins with receptor autophosphorylation and culminates in the tyrosine phosphorylation of a variety of cellular substrates and ultimately in the initiation of nuclear events necessary for the overall cell response. Schlessinger and Ullrich, Neuron 9:383-391, 1992. Individual phosphotyrosine residues of the cytoplasmic domains of receptors may serve as specific binding sites that interact with a host of cytoplasmic signaling molecules, thereby activating various signal transduction pathways, Ullrich and Schlessinger, Cell 61:203-212, 1990.
The intracellular, cytoplasmic, non-receptor protein tyrosine kinases do not contain a hydrophobic transmembrane domain or an extracellular domain and share non-catalytic domains in addition to sharing their catalytic kinase domains. Such non-catalytic domains include the SH2 domains (SRC homology domain 2; Sadowski et al., Mol. Cell. Biol. 6:4396-4408; Koch et al., Science 252:668-674, 1991) SH3 domains (SRC homology domain 3; Mayer et al., Nature 332:269-272, 1988) and PI domains (also called PTB domains Blaike, et al., JBC 269:32031-32034, 1994; VanderGeer and Pawson TIBS 20: 277-280, 1995). The non-catalytic domains are thought to be important in the regulation of protein-protein interactions during signal transduction. Pawson and Gish, Cell 71:359-362, 1992.
A central feature of signal transduction (for reviews, see Posada and Cooper, Mol. Biol. Cell 3:583-392, 1992; Hardie, Symp. Soc. Exp. Biol. 44:241-255, 1990), is the reversible phosphorylation of certain proteins. Receptor phosphorylation stimulates a physical association of the activated receptor with target molecules. Some of the target molecules such as phospholipase C.gamma. are in turn phosphorylated and activated. Margolis et al., Cell 57:1101-1107, 1989; Margolis et al., Science 248:607-610, 1990; Nishibe et al., Science 250:1253-1255, 1990; and Kim et al., Cell 65:435-411, 1991. Such phosphorylation transmits a signal to the cytoplasm. Other target molecules are not phosphorylated, but assist in signal transmission by acting as adapter molecules for secondary signal transducer proteins. For example, receptor phosphorylation and the subsequent allosteric changes in the receptor recruit the Grb-2/SOS complex to the catalytic domain of the receptor where its proximity to the membrane allows it to activate ras. Pawson and Schlessinger, Current Biol. 13:434, 1993.
Receptor phosphorylation is essential for binding and phosphorylation of cytoplasmic target proteins that contain SH2 domains, such as phospholipase C.gamma. (PLC.gamma.), p21.sup.ras GTPase-activating protein (GAP), phosphatidylinositol (PI) 3'-kinase (PI3K), p60.sup.arc and related tyrosine kinases, growth factor receptor-bound protein 2 (GRB-2), vav, SHC, CRK, NCK, and PTP1D, (also called SH-PTP2 or syp) and for proteins that contain PI domains, such as SHC. High affinity binding of these signal-transducing factors is strictly dependent on SH2 domains and tyrosine-phosphorylated, short sequence motifs within different domains of the receptor tyrosine kinase (Koch, C. A. et al., (1991), Science, 252, 668-674).
The secondary signal transducer molecules generated by activated receptors result in a signal cascade that regulates cell functions such as cell division or differentiation. Reviews describing intracellular signal transduction include Aaronson, Science, 254:1146-1153, 1991; Schlessinger, Trends Biochem. Sci., 13:443-447, 1988; and Ullrich and Schlessinger, Cell, 61:203-212, 1990. However, the search for receptor-specific signal transducers and regulators which (in addition to generally employed SH2 domain substrate proteins) define ligand- and cell type-characteristic effects has so far had only limited success.